Wang Yuanbin

Dynamic behavior of PWM DC-DC converters in starting process

Closed loop PWM DC-DC switching converter is strongly nonlinear, and the analytical solution is difficult to be achieved. Oscillation and impulse voltage will probably occur in the output during the starting period in DC-DC converters. In this paper, a Pulse-Width-Modulation(PWM) buck converter is used as an example to discuss such problems so as to predict them in the course of design, and reduce or even avoid above harmful phenomena by modifying the parameters. In order to investigate the dynamic behavior in the output voltage of the converters, differential equations for inductor current profile and output voltage profile are established. The equations are based on the assumption that the frequency of inductor current profile and output voltage profile are much lower than the switching frequency of the converter. The condition leading to oscillation and impulse voltage in the output is obtained. The oscillation frequency, the time that the impulse voltage occur and the max value of impulse voltage are also derived. The proposed approach is verified by PSPICE simulation and experiment results.

Fire Detection Based on Image Processing in Coal Mine

Fire detection is very important in safety area, and at all times it is a difficult problem in coal mine. In this paper, a real-time fire-detector algorithm is proposed. In early stage there is no evident flame, and the temperature is low, the visible light is scarce, so the signal is difficult to achieve by ordinary color CCD. Here infrared CCD is employed as a probe, and obtained infrared image is input the computer, then fire recognition is determined based on image processing. Firstly, the grey scale binary image is partitioned by entropy threshold method and then smoothed by Median filtering. Then significant characteristics of flame are extracted and after normalization the foregoing data are input neutral network for recognition. All of the above clues are combined to reach a final decision. Experiment results show that the proposed method is successful in detecting fire or flames. In addition, it reduces the false alarms remarkably. It is also shown that the method works in a variety of conditions. The system has high validity and accuracy.

Research on Dynamic Performance of Step-Down Converters with Input Perturbation

For PWM DC—DC converters, keeping the output voltage steady is very important. When the input voltage has a step change, up impulse or down impulse voltage will occur in the output. Both the magnitude and the duration time of the over voltage are severely influencing to the whole system, which will destroy or disorder the circuit. Step-down converter is used as an example to investigate the input perturbation phenomenon. Differential equations for inductor current profile and output voltage profile are established. The equations are based on the assumption that the frequency of inductor current profile and output voltage profile are much lower than the switching frequency of the converter. The condition leading to oscillation and impulse voltage in the output is obtained. The oscillation frequency, the time that the impulse voltage occurs and the max value of impulse voltage are also derived. Dynamic behavior of the PWM controlled buck converters in DCM is also discussed. The proposed method is tested by simulation. The simulation results show that the analysis is coherent with the analysis..

Research on Chaos in Switching Dc-Dc Converters

Switched-mode DC-DC converters are nonlinear and time-varying systems. Chaos may occur in some case and it will influence the working performance even the entire system greatly. So chaos should be investigated in detail. A nonlinear discrete modeling technique is employed here. The system is described in terms of a sequence of sample , one per switching cycle, involving a mapping from one sampling instant to the next. The variation of the state vector within in a cycle is described by a function and it can be applied iteratively to determine how the state of the converter evolves from cycle to cycle. The proposed approach is verified by simulation and experiments, respectively.

Dynamic Analysis of PWM Switching DC-DC Converters

The dynamic behavior of DC-DC converters is analyzed on the basis of a state-space averaged model including the feed back loop. The low frequency ripple behavior of DC-DC converters is analyzed in s field. A buck DC-DC converter is used as an example. The condition causing oscillation and impulse during starting period and in case of input perturbation is obtained. The output to input voltage transfer function in the feedback loop is deduced. The stability requirement is obtained based on Hurwitz stability criterion. The frequency and amplitude of the low frequency ripple voltage are determined by using zero-pole analysis method. The proposed approach is verified by simulation and experiment results